The 8 subtypes of metabotropic glutamate receptors (mGlu) are expressed heterogeneously throughout a myriad of brain circuits in different patterns, often with overlapping expression of receptor subtypes. While these receptors have been thought to be organized as disulfide-linked homodimers, a study in heterologous cells has shown that, when co-expressed, particular mGlu receptor protomers can heterodimerize. Whether this ability to interact is of any physiological relevance was previously unknown. Recently we teamed with the Conn/Niswender laboratories at Vanderbilt University to show conclusively that mGlu2 and mGlu4 protomers form functional heteromers with a unique pharmacology. Specifically, using complemented donor-acceptor resonance energy transfer (CODA-RET), a methodology that our laboratory developed to probe signaling from defined homodimers or heterodimers, we showed that certain mGlu4-selective positive allosteric modulators (PAMs), including PHCCC and ADX88178, act only at mGlu4 homodimers, whereas other mGlu4 PAMs, including VU0155041 and Lu AF21934, also act at mGlu2/4 heterodimers. Using these pharmacological tools, the Conn laboratory has shown that, in some brain circuits, such as the striatopallidal synapse discussed above, mGlu4/4 homodimers are the signaling entities, whereas in other pathways, such as corticostriatal and thalamocortical synapses, mGlu2/4 heterodimers play a more important role in signaling. This has established a new paradigm regarding the potential for mGlu heterodimer activity and has provided an exciting opportunity to use existing or novel ligands as selective tools to target heterodimers or homodimers for new therapeutic applications to reverse pathophysiological changes in critical circuits that contribute to psychotic disorders, epilepsy, cognitive dysfunction, Parkinson's disease and more. We have now identified examples of paradoxical pharmacology that we believe may be explained by heterodimerization of various other mGlu receptors. Specifically, we cite examples consistent with heterodimerization between mGlu4/7, mGlu7/8, and mGlu1/5. We will use CODA-RET to systematically explore the ability of these heterodimers to signal to G protein and then will test the hypothesis that available PAMs and NAMs exhibit distinct homodimer- and heterodimer-selective profiles through the following specific aims: 1) To define the homodimer- and heterodimer-selective actions of allosteric modulators of mGlu4, mGlu7 and mGlu8 receptors. 2) To define the homodimer- and heterodimer-selective actions of allosteric modulators of mGlu1 and mGlu5 receptors. The end results of this R21 will be a validated suite of compounds that can be used in subsequent studies to begin to dissect key brain circuits using these selective reagents ex vivo and in vivo.